1. Field of the Invention
This invention relates in general to improved rotary-percussion earth boring bits and in particular to improvements in the geometric relationship of the lower region of the bit and cutting inserts.
2. Description of the Prior Art
The first known commercially successful rotary-percussion earth boring bit of the type having button inserts is described in U.S. Pat. No. 3,185,228 which issued to the assignee of Joseph L. Kelly, Jr. on May 25, 1965. While such bits have been successful, often the outer or gage row of inserts wears out or break before inserts on the inner portions of the bit serve their useful life.
There are several explanations that may explain the variation between gage and inner row insert life. A greater amount of material must be cut by the gage or outer row inserts, which have a greater circumference to travel than the inner row inserts on each revolution of the bit. Greater forces are exerted on the outer row of inserts, for in addition to the vertical compressive forces, large lateral forces occur, resulting from the cutting of the sides of the borehole. The typically used tungsten carbide insert will withstand great compressive forces, but not great lateral or bending forces, thus breakage is more likely.
The outer row inserts are also more likely to be pulled from the support metal of the body by large radial tearing forces. The outer inserts protrude beyond the enlarged lower region of the bit to avoid excessive wear on the lower region surface and wedging. Because the outer inserts are close to the periphery of the bit, support metal is thin in this area. This leads to rupture of the support metal near the periphery.
A number of solutions to the problems of wear, breakage, and tearing for the outer row and their supporting metal have been proposed. However, improvements in the bit life are still needed.